This invention relates to the field of ballast circuits, and more particularly, this invention relates to ballast circuits using a ramped ignition frequency.
Ballast circuits are commonly used for operating a lamp to prevent the sudden, large increases in voltage supplied to the lamp that could result in malfunction or damage to the lamp. Ballast circuits also control operation of a lamp using a preheating mode and an operating mode. For example, a fluorescent ballast typically operates a fluorescent lamp using circuits known to those skilled in the art. These circuits usually provide for some form of filament preheating to extend the operating life of the lamp.
One prior art circuit for preheating the lamp filaments applies a high-frequency current at a low voltage level. As the filaments are heated, the emissions from the filaments help to lower the voltage at which the lamp will ignite. This voltage at which a lamp ignites depends on many factors, including the lamp type, the preheating of the filaments, and the ambient temperature. For example, ballast control integrated circuits are commonly used and have varying methods to control the preheating time, oscillator frequency, the ignition frequency, and final operating frequencies.
These prior art circuits, however, do not provide adequate control over the preheating and ignition profile, which is important for operating efficiency among different types of lamps.
It is therefore an object of the present invention to provide a ballast circuit having a programmable intermediate frequency.
It is another object of the present invention to provide a ballast circuit that overcomes the disadvantages as noted above.
The present invention advantageously changes an ignition frequency ramp profile for a ballast circuit from a linear ramp to a ramp that has a programmable intermediate frequency and an adjustable time period. In accordance with one aspect of the present invention, the ballast circuit for operating a lamp includes a lamp preheat/ignition circuit for preheating and igniting the lamp. A ballast controller integrated circuit is operatively connected to the preheat/ignition circuit, wherein the lamp preheat/ignition circuit is operably controlled in a:
a) preheating mode wherein the lamp is preheated at a preheating frequency for a predetermined period of time;
b) a user programmable intermediate ignition mode wherein the lamp is heated at an intermediate ignition frequency that is lower than the preheating frequency; and
c) an operating mode wherein the lamp is operated at a final operating frequency that is lower than the intermediate ignition frequency.
In yet another aspect of the present invention, the lamp preheat/ignition circuit includes a preheat capacitor that is operatively connected to the ballast controller integrated circuit. This preheat capacitor is charged at a constant current during the preheating mode. The preheat capacitor is discharged and charged a second time at a different constant current from that used in the preheating mode during which the frequency drops from the intermediate ignition frequency to the final operating frequency in a linear manner. The lamp preheat/ignition circuit also includes at least one capacitor and a pair of operational amplifiers operative with the preheat capacitor and having an inverting input and an output for switching among the preheating, intermediate ignition, and operating modes.
In yet another aspect of the present invention, the lamp preheat/ignition circuit includes a preheat capacitor and differential amplifier circuit. An operational amplifier circuit is operatively connected to the ballast controller integrated circuit and lamp/preheat ignition circuit for resetting operation of the ballast circuit.
A method aspect of the present invention is also disclosed for operating a ballast circuit for a lamp by charging the preheat capacitor to establish a preheating time period for the lamp at a preheating frequency and discharging the preheat capacitor while establishing a user programmable intermediate ignition frequency for the lamp that is lower than the preheating frequency. The preheat capacitor is discharged and then recharged and a final operating frequency for the lamp established that is lower than the intermediate ignition frequency.
The method also includes the step of initially charging the preheat capacitor at a preheating frequency at a constant current, discharging the preheat capacitor, and recharging the preheat capacitor at a constant current different from the current used to charge initially the preheat capacitor. The frequency is lowered in a linear manner from the intermediate ignition frequency for the lamp to the final operating frequency during the recharging of the preheat capacitor.
In yet another aspect of the present invention, the preheat frequency can be set at a maximum programmed oscillator frequency that is output from the ballast controller integrated circuit. The final operating frequency can be set as a minimum programmed oscillator frequency that is output from the ballast controller integrated circuit. The preheating time period can be established by the time period it takes for the capacitor to charge up to a predetermined voltage. The ballast circuit can be reset using a operational amplifier circuit. The final output frequency can be established by a reference within the ballast controller integrated circuit with the use of additional passive components tied to the controller.